Correction: The Magic Number is Exactly 32

Because the ratio I measured a few days ago was a strange non-integer ratio, I decided to open up the gearbox and count teeth. It was frustratingly messy, to say the least.

Inside the wheelchair motor gear box. Lots of grease! This was after I had taken out about 80% of all the grease.

I was hoping to be able to count the number of teeth on each gear without taking the whole thing apart, but this proved quite difficult. There’s not really a good way to mark a tooth when the whole thing is swimming in grease.

After a while, I decided to sit down and do some math. It was a flashback to my old engineering school days. I was able to count the teeth on some of the gears (with a fair degree of confidence) and using this information, the problem statement became:

A gearbox has a gear ratio of 31.8. There are four gears in the box. The output gear has 32 teeth. The gear connected to the output gear has 11 teeth. A second gear with an unknown amount of teeth is rigidly attached to the same shaft as the gear with 11 teeth. This gear is then driven by a worm gear connected to the motor. How many teeth are on the second gear?

With most word problems, I find myself drawing pictures to make things more clear. I made a simple CAD model of this setup to try and get a better grip on how the gears worked together.

A simple model of the gears in the gear box. The left gear has the output shaft with the tire on it, and the lower right cylinder represents the worm gear attached to the motor.

Assign a letter to each gear in the system. The gear on the output shaft is A, the gear meshing with it is B, the gear on the same shaft is C, and the worm gear in the lower right is D.

The total ratio is the ratio between A and B times the ratio between B and C times C. Remember, one revolution of a worm gear will advance the corresponding spur gear one tooth, or 1 divided by the number of teeth on the gear. Putting it all together:

The composite gear ratio for the wheelchair gearbox.

Wait a minute… that’s just a fancy way of saying the gear ratio is A, or the number of teeth on the output shaft gear. That can’t be right. Or can it?

Remember we measured the gear ratio at 31.8. I did some more estimating and came up with a number closer to 31.9 by counting more tire revolutions (320, or so I thought), but because I can’t exactly line the tire up in the same spot it started from, there was always going to be a little error in the calculation.

And come to think of it, the more tire rotations I counted, the closer my estimation came to 32 exactly. To come up with the 31.8 number I counted 159 revolutions. I was off by one tire rotation. And to come up with the 31.93 number I counted 639 rotations, which meant I was off by about 1.25 tire rotations. Turns out I’m not very good at counting.

At this point you might be wondering why I didn’t use the other encoder to measure the gear ratio with much higher precision (and without all the mess). The output shaft on the gearbox is 17mm diameter, but I mistakenly thought it was 8.5mm. I misread the radius measure in my CAD program and thought it was a diameter measure instead. You can jury rig a 10m ID encoder to fit on an 8.5mm shaft, but not a 17mm shaft.

Long story short, I like to do things the hard way, and the magic number is actually 32. Exactly 32.